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Community and Q&A

Adding extra insulation under the house

Randy Garrett | Posted in General Questions on

Hi, I have two questions about adding some extra insulation under my house. Our house has R-19 under the whole house. With 2×10 floor joist, there is still plenty of space for extra insulation. I have some unfaced R-19 left from my parents house. My first question is: Can I just add another layer of insulation right on top of the existing insulation? The second is: Would there be a problem with putting Tyvek on the floor joist to support the extra layer of insulation? Thanks for your help.


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  1. Aj Builder, Upstate NY Zone 6a | | #1

    Read this and your head may explode with joy or questions, but this is some of the best work I have ever seen coming from BSC. I met Joe years ago. Amazing gent. Great story teller. Go to hear him speak. You will be most pleased.

  2. GBA Editor
    Martin Holladay | | #2

    1. It's always a good idea to tell us your location or climate.

    2. You forgot to tell us whether you are insulating the floor of a building on piers, or a floor over a basement, or a floor over a crawl space.

    3. It never hurts to fill joist bays completely with fiberglass batts, although fiberglass batts often perform poorly.

    4. You need something more durable than Tyvek to protect the fiberglass batts, especially if the house is on piers (in which case you'll need to protect the fiberglass with OSB or plywood).

    5. The performance of the insulation will be greatly increased if you install a continuous layer of polyisocyanurate (rigid foam) insulation under your joists. The seams should be carefully sealed with canned foam or tape. If your house is on piers, the foam will still need to be protected with OSB or plywood.

  3. Randy Garrett | | #3

    Thanks for the answers! I live in Richmond, VA. The area in question is a crawl space. I would be able to use the rigid foam on about half of the house, but the rest has too many water lines,etc. in the way.
    I did notice that there is no plastic under the all the large heating ducts that run from the unit located in the garage. Would this a good place to try and slide some of the rigid foam or plastic under? It looks like there is about 3 inches of space. I am going to put a new layer of plastic under the whole house and run it about 2 feet up the foundation. I am leaving the original layer of plastic and covering it with the new. Does this sound ok?

    Thanks, Randy

  4. Aj Builder, Upstate NY Zone 6a | | #4

    Randy, more info, I think since only your eyes see all your details you will have to read up and make some choices. Both of my postings should help. Lastly, you can check your own work with low cost digital temperature, humidity meters from Lowes for example. I use them and they are very helpful. I leave them in customers homes and check min max seasonally to make sure all is well. I also visually check and use my hand and a gun style IR temp meter.

    All of the wood in your floor needs to be dry and free from stuff growing on it and or dry rot. The humidity has to be below 60% is what I shoot for. I have some basements that run dehumidifiers to maintain safe levels. New ones that are made for basements work well for me. They are energy hogs though and best to not need them. Same for sump pumps and radon mitigation set ups. Sometimes they are necessary.

  5. GBA Editor
    Martin Holladay | | #5

    I don't know what you mean when you write, "I did notice that there is no plastic under the all the large heating ducts." What kind of plastic are you talking about? Are you talking about a polyethylene ground cover on the dirt floor? Or are you talking about a polyethylene jacket protecting duct insulation?

  6. Randy Garrett | | #6

    I was talking about the ground cover on the dirt floor, not the duct insulation. It's all the large box-like duct work. It is just a few inches off the ground. The main trunck line and lines off of it are up near the floor. I didn't know if putting some type of protection under the large duct work would be necessary or beneficial.
    There are a few spots where the contractors missed with the plastic ground cover. All the area between the piers under the house and not all the way to the outside walls. I am getting ready to put down another layer of new plastic under the florida room area. I'll take pictures and if I can post them, maybe you can see what it looks like and give some advice on what else can be done.


  7. Steve El | | #7

    Have you considered turning the crawlspace into conditioned space?

    This might be of interest

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